A Turkish defence ministry official said a ground operation into Syria was one option for Turkey, which has previously mounted several incursions into northern Syria against the Syrian Kurdish YPG militia.

The YPG is a central element of U.S.-allied forces in a coalition against Islamic State. Support for the YPG by the United States and other allies, including France, has strained ties with Ankara.

Turkey says the YPG is a terrorist organisation, closely tied to the Kurdistan Workers Party (PKK) militant group. Turkey's Western allies list the PKK as terrorist group, but not the YPG.

BORN IN SYRIAN WAR

The YPG, or the People's Protection Units, emerged as a powerful armed group during the Syrian civil war, which began in 2011. It established a foothold in the north as Syrian government forces withdrew to put down the uprising against President Bashar al-Assad elsewhere. It is affiliated to the main Syrian Kurdish faction, the Democratic Union Party (PYD), and has a female counterpart, the YPJ.

YPG control was initially concentrated in three predominantly Kurdish regions of northern Syria - known in Kurdish as Rojava. The area is home to roughly 2 million Kurds and Kurdish-led authorities have established autonomous governing bodies there since the start of the Syrian war.

A U.S. ALLY

The YPG's influence expanded as it allied with the U.S.-led coalition against Islamic State, becoming the spearhead of a broader group, the Syrian Democratic Forces (SDF), which played a key role defeating the jihadists across Syria.

As Islamic State retreated, the area of SDF control grew, and now accounts for around one quarter of Syria, including oil fields and several mostly Arab areas.

The U.S.-led coalition says it continues to support the SDF.

WHY DOES TURKEY SEE THE YPG AS A THREAT?

Turkey views the PYD and YPG as indistinguishable from the PKK, which launched an insurgency in Turkey for Kurdish political and cultural rights in 1984.

More than 40,000 people have been killed in the conflict.

Turkey's Kurdish minority amounts to 15-20% of its total population of around 85 million, mostly living in eastern and southeastern areas bordering Syria. Wary of separatism, Turkey views the PYD's Syrian foothold as a national security threat.

The YPG is heavily influenced by the ideas of PKK leader Abdullah Ocalan, who has been in jail in Turkey since 1999, convicted of treason.

The PKK is designated a terrorist organization by Turkey, the United States and the European Union.

Western states, including Turkey's NATO allies, do not view the YPG as a terrorist group. U.S. support for the SDF has been a source of tension with Turkey for years.

With support from Syrian Arab insurgent groups, Turkey's previous incursions into northern Syria have brought swathes of territory under its control, including the Afrin region, which was previously one of three main Kurdish areas.

TENSE TIES WITH DAMASCUS

Syria's Baathist state systematically persecuted the Kurds before the war. Yet the YPG and Damascus have broadly stayed out of each other's way during the conflict, notwithstanding occasional clashes. They also have shared foes, including the Sunni Arab groups backed by Turkey.

The YPG has allowed the government to keep a foothold in its areas, including control of Qamishli airport.

Kurdish leaders say their aim is regional autonomy within a decentralized Syria, not independence. But Damascus opposes Kurdish autonomy demands, and talks between the sides for a political settlement have made no progress.

KURDISH GROUPS IN THE REGION

The YPG and PYD are the main political grouping of Kurds in Syria but there are Kurdish groups with different loyalties spread across the region.

The ruling party of the Kurdish Regional Government of northern Iraq has close ties with Turkey, which it depends on to export oil, but its rival partner in government is more critical of Ankara.

The PKK has bases in the mountain ranges of northern Iraq.